BA at LGW, generally
I think the real question is BA's future at LGW; we know that Willie Walsh has given them about a year to shape up - or else; my expectation is that the airline will ultimately decide to up stakes and leave, particularly if an Open Skies deal is done. Then, BA will be allowed to move most of its LGW long haul flights to LHR (and no doubt shaft a few short haul, including domestic). At that stage, short haul flights won't be of much interest as connecting flights, so they'll gradually wind it down.
Let's be honest, although (in my experience) the BA 735s are perfectly nice aircraft, they're not getting any younger and I simply can't see the airline investing billions in new Airbuses for what are essentially "surplus to requirement " routes. On top of that, you have the issue of BA's GB Airways franchise. WW is apparently against franchises and there have been various rumours about the future of this.
What it comes down to is that ultimately, BA will not be flying JER-LGW. It may not be this year or next year, but it will happen. And thank goodness, for once, we've preempted what could have been a serious blow to our air links by getting back the LHR route.